In my prior application, Ser. No. 942,599 filed on Sept. 15, 1978, now U.S. Pat. No. 4,222,076, there were disclosed apparatus and method for encoding, transmitting and reconstructing gray-scale pictures by a hierarchical scheme of successive binary splittings of image space. At each hierarchical level gray values were rendered as useful approximate representations of the overall image in progressively finer detail. It was thus possible to reproduce the original image exactly in a progressive manner rather than in line-by-line fashion without paying any penalty in number of bits transmitted. Due to the early recognition of the nature of an image progressively transmitted, a degree of compression of time was realized when browsing through a series of images.
With images whose ultimate picture elements or pixels are simply white or black, herein called binary, compression of transmission time becomes an important consideration because there are typically large monotone background areas. Representative compression ratios using sophisticated blob encoding techniques, such as have been disclosed by A. J. Frank in U.S. Pat. No. 4,103,287 issued July 25, 1978 and entitled "Variable Length Codes for High Quality Image Encoding", are in the range of ten or twenty to one. This means that the number of encoding bits required to specify an image is only five to ten percent of the number of pixels in the image space. The Frank method achieves high compression ratios by identifying and stating the positions of "blobs"of monotone material and then encoding only the location of the leading and trailing edges, thereof, in each scan line. The total image is transmitted and reconstructed either blob by blob or in one top-to-bottom scan which develops blobs on the same level in parallel.
What is desired is a progressive picture display which at the same time achieves a significant compression. The basic advantage of progressive display of binary pictures over line-by-line display is that in early stages of reconstruction one sees the entire image in crude form, instead of seeing only the top part in final form.